Purification of fluorocarbons



rosion resistance and insulating properties.

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3,022,357 PURIFICATXON F FLUOROCARBONS Klaus Bernd Kasper, Vienna, W. Va., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Oct. 8, 1959, Ser. No. 845,075

7 7 Claims. (Cl. 260653.3)

The process of the present invention relates to the "purification of fluorocarbons and, more particularly, to

formation of low molecular weight products which may not be suitable for use as plastics.

One of the preferred methods or preparing unsaturated fluorocarbon compounds is by pyrolysis of chlorodifluoromethane at temperatures above 700 C., as disclosed in 'greater'detail in U.S. Patent 2,551,573 which issued to Downing et al. on May 8, 1951. The process disclosed in this patent results in a large number of fluorocarbons "in addition to'tetrafiuoroethylene and hexafiuoropropylene.

Hexafiuoropropylene isolated by distillation from this pyrolysis process was found to contain small quantities of perfluorobutyne-2 which detrimentally aifected the rate of polymerization, as well as the degree of polymerization of hexafluoropropylene, particularly in copolymerization with other ethylenically-unsaturated monomers such as tetrafiuoroethylene. This impurity could not be readily removed by standard distillation techniques in view of the close boiling points of hexafluoropropylene and perfluorobutyne-Z at atmospheric pressure.

It is therefore one of the objects of the present invention to provide an improved method for preparing hexafluoropropylene. It is another object of the present invention to provide a method for the purification of hexafiuoropropylene. Still anotherobject is the separation of hexafluoropropylene from perfiuorobutyne-Z. v 1

The objects of the present invention are accomplished by a process which comprises reacting a mixture of hexafiuoropropylene and perfiuorobutyne-2, said mixture containing the perfluorobutyne-2 in a concentration of 0.001 to by weight of the 'hexailuoropropylene, with ammonia in a liquid phase, selected from the class consisting of water and liquefied reagents, the concentration of said ammonia being from 1 to 30 weight percent of the water when said liquid phase is water, and from 0.01 to 1.5 weight percent when said liquid phase comprises the liquefied reagents, and recovering hexafiuoropropylene from the reaction mixture containing a substantially lesser amount of perfiuorobutyne-Z than present in the original mixture.

Hexafluoropropylene, when prepared by the pyrolysis of chlorodifluoromethane, contains anywhere from 0.0.01

3,022,357 Patented Feb. 20, 1962' 2 to 10% of perfluorobutyne-Z, which, as described, deleteriously affects the polymerization of hexafluo'ropropylone. The present invention is based on the discovery that perfluorobutyne-Z, in presence of hexafluoropropylene, reacts rapidly with ammonia to form products which are liquid or solid at temperatures below 50 C., and which are water-soluble. Thus, perftuorobutyne-Z can be removed from a hexafluoropropylene stream by reaction with ammonia followed by water scrubbing of 10 the reaction product." Ammonia also reacts with hexafluoropropylene to result in a mixture of aminated products; however, the rate of the reaction of hexafluoropropylene with ammonia is substantially lower than the rate of reaction of perfiuorobutyne 2, so that under favorable conditions yield losses of hexafiuoropropylene due to reaction with ammonia are insignificant. The products formed by the reaction of hexafiuoropropylene with ammonia, are soluble in water and are readily removed by water scrubbing. Favorable reaction conditions are achieved when the reaction of ammonia and the fluorocarbon mixture is carried out in a liquid phase in which the reactants are homogeneously admixed.

In general, the purification of hexafluoropropylene, in accordance with the present invention, is carried out by two'rnajor techniques. In the anhydrous and preferred method, gaseous hexafluoropropylene containing perfluorobutyne-2 is admixed with anhydrous gaseous ammonia.

The mixture is condensed and allowed to remain in the liquid state for a sufiicient length of time to allow complete reaction of the perfluorobutyne-2 with-the ammonia. It was found that neither reaction in the gas phase nor mixing of the reagents in the liquid phase gives rise to the desired purification. In the gas phase, the reaction is extremely slow While with the. liquid phase mixing,

the reaction is too fast to allow the preferential reaction 7 of ammonia with perfiuorobutyne-2 to go to completion before reaction with hexafiuoropropylene occurs. The reaction of ammonia with perfiuorobutyne-2 while keeping the reaction of hexafluoropropylene with ammonia to a minimum is, however, accomplished when the reagents are admixed in the gas phase and then simultaneously condensed to the liquid, phase. Upon reaction, pressure and/or temperature are changed .so as to allow vaporization of the hexafiuoropropylene. The hexafluoropropylene is then water-scrubbed to remove the byproducts in the gas stream.

Using this method, it was found that up to 99% of th perfluorobutyne-Z impurity can be removed. The degree of purification partially depends on the quantity of the ammonia employed in the reaction. Thus,- at least a stoichiometric quantity of the ammonia, based on the quantity of perfiuorobutyne-Z, should be employed. In general, the concentration of ammonia is varied between 0.01 to 1.5 weight'percent or 0.1 to 12 mol percent on the basis of hexafiuoropropylene. Within this range an increase in concentration of NH will result in a higher degree of purification. Still higher concentrations result in only a slight further increase in perfluorobutyne-Z removal. Such increases are generally not warranted. since the remaining perr'iuOrobutyne-Z in the hexafluoropropylene does not exert any apparent effect on the polymerforming ability of the monomer. Excess quantities are undesirable in that the excess ammonia can react with the hexafluoropropylene to decrease the hexafiuoropropylene recovered. The yield loss of hexafiuoropropylene employing the above-described method is, in general, less than 0.5%. The purification process as described hereinabove is generally carried out at temperatures ranging from 40 to +50 0., although such temperatures are not critical from the standpoint of operability of the process. At temperatures below 40 C., the reaction rate of perliuorobutyne-Z with NH is too slow to make the purification economically attractive. Temperatures above 50 C. are, although feasible, not generally employed, since the liquefaction of the gases involved in the process of the present invention becomes more diflicult as the temperature is increased. Pressure employed is similarly not critical, but should be at least sufiicient to maintain the reagents in the liquid state at the reaction temperature.

in a second method of carrying out the process of the present invention, gaseous hexafluoropropylene containing perfluorobutyne-Z is passed through an absorptiontower containing an aqueous solution of ammonia. The removal of periluorobutyne-Z is greatly improved by the presence of noble metals, including copper, which appear to act as catalysts for the reaction of perfitiorobutyne-Z with NH without catalyzing the reaction of hexafiuoropropylene with NH The concentration of the ammonia may be varied from 1 to 30 weight percent or higher, based on water, and is not critical. The quantity of noble metal employed is not critical and may be greatly varied. The temperature is generally maintained between and 50 0; higher or lower temperatures may, however, be employed. Suflicient positive pressure is maintained to pass the hexafiuoropropylcne through the tower.

Amines, such as dimethyl amine, also react with perfiuorobutyne2 to form analogous products. However, the rate differential between the reaction of the amine with perlluorobutyne-Z and the reaction of the amine with hexafiuoropropylene is not large enough to allow etficient Example I A sample of hexafiuoropropylene containing 6102 p.p.m. of perfluorobutyne-Z was mixed with 5 mol percent of anhydrous ammonia. The gas mixture was then passed into a capillary tube maintained at Dry Ice temperature. The tube was sealed and warmed to 30 C. under autog enous pressure for a period of 60 minutes. The reaction mixture was then allowed to vaporize and was washed with water. Analysis of the resulting hexafiuoropropylene showed a perfiuorobutyne-Z content of less than p.p.m. No other detectable impurities were found.

Example II Employing the procedure set forth in Example I, a hexaliuoropropylene stream containing 537 p.p.m. of perfiuorobutyne-Z was mixed with 1.5 mol percent of anhydrous ammonia in the gas phase, condensed, and reacted for 5 minutes at 30 C. Analysis of the washed hexafiuoropropylene gas showed a perfluorobutyne-Z content of less than 10 p.p.m. and no other detectable impurities.

Example III Employing the procedure set forth in Example II, a

hexafluoropropylene stream containing 1200 p.p.m. of

perfiuorobutyne-Z was mixed with 1.5 mol percent of anhydrous ammonia in the gas phase, condensed, and reacted for 3 minutes at 0 C. Analysis of the washed hexafiuoropropylene showed a perfiuorobutyne-Z content of 60 p.p.m. and no detectable other impurity.

Example IV Metered flows of hexafiuoropropylene containing 300 of hexafluorobutyne-Z (1.5 ml./min. at standard conditions), and anhydrous ammonia (25 mL/min. at standard conditions) were mixed in the gas phase at room temperature and condensed in a A in. ID. stainless steel tubing of one ft. length. The temperature of the tube was maintained at -l5 C. to -20 C. The liquid mixture was then passed into a tubular reactor maintained at 0 C. and at 45 p.s.i.g. Hold-up time of the reaction mixture in this reactor was 5 minutes. After passing through the reactor, the resulting liquid was vaporized and passed into a water-scrubber. The hexafiuoropropylene effluent from the water scrubber contained 60 p.p.m. of perfluorobutyne-Z.

Example V Hexafluoropropylene, containing 1000 p.p.m. of perfiuorobutyne-Z, was passed at a pressure of 45 p.s.i.g. into an absorption tower at room temperature containing a 30% concentrated aqueous solution of ammonia and inch steel packing. The hexafiuoropropylene gas was passed at a space velocity of 0.63 g./hr./cc. of absorbing liquid for a contact time of approximately 0.05 minute. The efliuent hexafiuo-ropropylene contained 332 p.p.m. of perfluorobutyne-2. The yield loss of hexafiuoro-propylene as indicated by fluoride analysis of the ammonia solution was 2.75%.

Example VI Hexafluoropropylene containing 1000 p.p.m. of per fluorobutyne-Z was passed at a pressure of 45 p.s.i.g. into an absorption tower containing a 5.5% aqueous solution of ammonia and packed with copper metal. The boxafluoropropylene was passed at a space velocity of 1.3 g./hr./ml. of absorbing liquid for a contact time of approximately 0.05 min. The efiluent hexafiuoropropylene contained 32 p.p.m. of perfiuorobutyne-Z. The yield loss of hexafluoropropylene, as indicated by fluoride analysis of the ammonia solution, was 0.38%.

The examples have illustrated certain embodiments of the present invention, and are not to be construed as limiting the invention. Various process modifications will be apparent to those skilled in the art.

Hexafluoropropylene purified by the process of the present invention has improved chemical reactivity in the field of polymerization, as well as in the field of compound synthesis.

I claim:

1. A process of separating perfiuorobutyne-2 from hexafluoropropylene which comprises reacting a mixture of hexafluoropropylene and perfluorobutyne-Z with ammonia in a liquid phase selected from the class consisting of water and liquefied reagents, the concentration of said ammonia being from 1 to 30 weight percent of the liquid phase when said liquid phase is water, and from 0.01 to 1.5 weight percent when said liquid phase comprises the liquefied reagents, and recovering hexafluoropropylene.

2. The process set forth in claim 1 wherein the concentration of the perfluorobutyne is from 0.001 to 10% by weight of the hexafluoropropylene.

3. A process of separating perfluorobutyne-Z from hexafluoropropylene which comprises forming a mixture of gaseous hexafluoropropylene and perfiuorobutyne-Z, said mixture containing the perfluorobutyne-Z in a concentration of 0.001% to 10% by weight, with from 0.01 to 1.5 weight percent of gaseous anhydrous ammonia, condensing the resulting mixture to the liquid state at a temperature of -40 to +50 C. for a time sutficient ot allow reaction of ammonia and perfluorobutyne-Z and recovering hexafiuoropropylene from the reaction mix ture through vaporization. i

4. The process as set forth in claim 3 wherein the recovered hexafiuoropropylene is water-scrubbed.

5. A process for separating perfluorobutyne-2 from hexafluoropropylene which comprises passing a gaseous mixture of hexafiuoropropylene and perfiuorobutyne-Z, said mixture containing perfluorobutyne-Z in a concentration of 0.001 to 10% by weight, through an aqueous solution of ammonia, containing the ammonia in a concentration of 1 to 30% by Weight of water at a temperature of 0 to 50 C.

6. The process set forth in claim 5 in the presence of noble metal.

7. The process set forth in claim 5 in the presence of copper.

References Cited in the file of this patent UNITED STATES PATENTS 2,615,926 Benning et a1 Oct. 28, 1952 2,691,036 Miller Oct. 5, 1954 2,722,558 Johnston Nov. 1, 1955 2,723,297 Litant et a1 Nov. 8, 1955 2,729,687 Sterling Ian. 3,1956 2,831,902 Hamilton et al Apr. 28, 1958 2,889,378 Boettger et a1. June 2, 1959 

1. A PROCESS OF SEPARATING PERFLUOROBUTYNE-2 FROM HEXAFLUOROPROPYLENE WHICH COMPRISES REACTING A MIXTURE OF HEXAFLUOROPROPYLENE AND PERFLUOROBUTYNE-2 WITH AMMONIA IN A LIQUID PHASE SELECTED FROM THE CLASS CONSISTING OF WATER AND LIQUEFIED REAGENTS, THE CONCENTRATION OF SAID AMMONIA BEING FROM 1 TO 30 WEIGHT PERCENT OF THE LIQUID PHASE WHEN SAID LIQUID PHASE IS WATER, AND FROM 0.01 TO 1.5 WEIGHT PERCENT WHEN SAID LIQUID PHASE COMPRISES THE LIQUEFIED REAGENTS, AND RECOVERING HEXAFLUOROPROPYLENE. 